Per Brian Robb of 98.5 The Sports Hub in Boston, Turner Sports' David Aldridge said on NBA TV that getting Crowder is a must if Chicago is going to consider dealing Butler to the Celtics.
ESPN.com's Zach Lowe wrote Monday that it seems unlikely Boston and Chicago will come to terms on a deal involving Butler.
K.C. Johnson of the Chicago Tribune added Tuesday that the Bulls have told teams they have no interest in trading Butler.
Boston's interest in Butler dates back to the night of the 2016 NBA draft, per Adrian Wojnarowski of The Vertical.
The Celtics are second in the Eastern Conference at 37-20, and adding a player of Butler's caliber could make them a true threat to the Cleveland Cavaliers.
Chicago is clinging to the seventh spot in the Eastern Conference at 28-29, and dealing Butler would essentially mean waving the white flag on 2016-17 even if it could manage to pry Crowder from Boston.
Butler still has two seasons remaining on his contract with a player option for 2019-20, plus he is in his prime at 27 years of age.
The former Marquette standout is averaging a career-high 24.5 points, 6.3 rebounds and 5.0 assists per game.
Crowder, who is Butler's former college teammate, is enjoying a strong season in his own right with 13.8 points and 5.2 boards per contest.
While Crowder's ruggedness on the defensive end makes Boston a difficult team to play against, it may need another superstar like Butler to pair with Isaiah Thomas in order to be a true championship threat.
The NBA trade deadline is Thursday at 3 p.m. ET.